Catheters have gradually evolved from single lumen catheters having a circular internal cross section to multiple lumen catheters which still use the circular cross section as the basic starting point for the shape of the outside of the catheter as well as the individual or combined shape of the multiple lumens. The use of the circular cross section lumen for a single lumen catheter provides the optimal flow characteristics because the frictional resistance to fluid flow through a circular lumen is minimized and there are no flow restrictive areas in the lumen. Additionally, laminar flow will occur through the lumen when a greater flow pressure is applied.
The designs for multiple lumen catheters have been created by splitting the circular cross section into multiple lumens. For example, one double lumen design which is used in the hemodialysis industry is the Circle-C lumen design. This cross sectional design involves reducing the cross sectional area of the main lumen and forming a second lumen concentrically along a portion of the outer surface thereof such that the overall shape of the catheter remains circular and the inner septum of the catheter looks basically like an eye brow. The most commonly used double lumen design in the hemodialysis industry is known as the Double-D lumen design. This design involves bisecting the circular cross section of a single lumen catheter to form two equal D-shaped lumens. Of these designs, only the Double-D design is believed to consistently provide flow rates for bi-directional blood flow of at least 250 ml per minute with a pressure gradient of less than 100 mm of mercury for a given outer diameter or French size.
The deficiencies inherent in the Circle-C designs are believed to be directly related to the existence of large flow restrictive and non laminar flow areas in the outer lumen due to the existence of sharp corners. Additionally, these designs include an increased amount of internal wall area which increases the amount of frictional resistance encountered by the fluid as it flows through the lumens of the catheter. The sharp corners of the Circle-C design create areas of lower or non laminar flow which increase the turbulence of fluid flow through the lumen as the velocity increases and may result in an increased likelihood that clots or other types of adhesions will form along the wall of the lumen in addition to the increased likelihood of hemolysis of the red blood cells occurring. To a lesser extent, the Double-D design has many of the same deficiencies present in the Circle-C design. For example, the Double-D design includes smaller flow restrictive areas than the Circle-C design although the flow of fluids through the lumens is still less than optimal due to the presence of lower or non laminar flow areas in the lumens adjacent to the intersection of the septum and the circular walls.
A further difficulty which arises with the Circle-C design and, to a lesser extent, with the Double-D design, is the deflection of the septum when the catheter is used for hemodialysis. In Hemodialysis, fluids are simultaneously infused and removed from the patient at high flow rates. With Double-D or Circle-C designs, fluid flows through one lumen in one direction and through the other lumen in the opposite direction. This bi-directional flow of fluids in the catheter is known to cause the deflection of the septum so that the flow area for fluid through one lumen is greater than in the other lumen and as one lumen becomes smaller than the other, the flow through the smaller lumen is further restricted for a given pressure.
Yet another difficulty which arises with the current Circle-C design and, to a lesser extent, with the Double-D design, is the reduction of flow when the catheter is bent or curved. When the catheters of these designs are bent or curved, the septum and, possibly the outer wall, of these catheters may deform to further restrict the flow of fluids through the lumens. One approach to solving the problem of septum deformation is disclosed in U.S. Pat. No. 5,221,255 granted to Mahurkar. In the Mahurkar patent, a separate rigid or semi-rigid member is disclosed to strengthen the septum of the catheter. In U.S. Pat. No. 5,348,538 granted to Young et al., the problem of septum deflection is addressed by coextruding the catheter with a plurality of materials having different durometers. In each of these designs, a significant and potentially expensive or time consuming modification of existing manufacturing methods and procedures may be required. In U.S. Pat. No. 5,156,592 granted to Martin et al., the concerns regarding septum deflection and wall collapse are addressed by the provision of a precurved catheter which is curved to conform to the shape of the incision typically formed in the body of the patient.
The present invention provides an optimal balance between the need to maximize the cross sectional area of the lumen of a catheter while minimizing the existence of sharp corners or flow restriction areas in the lumens. Similarly, the present invention provides an optimal balance between the need to minimize the occurrence of septum or wall deflection while providing a catheter with the smallest possible outer diameter for the intended use.